My guess is the Aschaffenberg-Würzburg line, also known as Main-Spessart-Bahn. As mentioned it stretches just around 90 kilometres. The line is primarily served by hourly RegionalExpress services, which were previously served by locomotive-powered push-pull double decker coaches. They were in 2018 mostly replaced by the Bombardier Twindexx Vario, which are 3-4 car self-powered double decker consists. Additionally to this, the line is served between Würzburg and Gemünden by RegionalBahn services formed of BR 440 trains, calling at stops that the RE skips. This part of the line is in rush hour supported by additional RB services. That is some info about the different traffic the line is represented by, and again this is just speculation based on the 'evidence' as to whether this is the line or not.